Capacitor discharge ignition (CDI) systems are widely used in spark-ignited internal combustion engines. Generally, CDI systems include a main capacitor, which during each cycle of an engine, is charged by an associated generator or charge coil and is later discharged through a step-up transformer or ignition coil to fire a spark plug. CDI systems typically have a stator assembly including a ferromagnetic stator core having wound thereabout the charge coil and the ignition coil with its primary and secondary windings. A permanent magnet assembly is typically mounted on an engine flywheel to generate current pulses within the charge coil as the permanent magnet is rotated past the ferromagnetic stator core. The current pulses produced in the charge coil are used to charge the main capacitor which is subsequently discharged upon activation of a trigger signal. The trigger signal is supplied by a trigger coil that is also wound around the stator core, wherein the permanent magnet assembly cycles past the stator core to generate pulses within the trigger coil. Upon receipt of the trigger signal, the main capacitor discharges through the primary winding of the ignition coil to induce a current in the secondary winding that is sufficient to cause a spark across a spark gap of the spark plug to ignite a fuel and air mixture within a combustion chamber of the engine. The time and occurrence of CDI is of importance to startability, output power, and emissions performance of engines, including small two and four stroke engines. Optimum ignition timing varies, primarily as a function of engine speed and engine load factors. Secondary factors, such as emissions performance and fuel quality, also play a role in determining optimum spark timing.
Microprocessor electronic timing control systems have been proposed for large engine applications, such as automotive engines, but typically are not well-suited to small engine applications because of cost and packaging constraints. Specifically, it has been proposed to employ microprocessor ignition modules in small engine applications, in which engine timing factors and desired advance or retard timing characteristics are pre-programmed into the microprocessor. For example, a microprocessor may be used to create a timing advance with increasing engine speed. However, cost constraints associated with microprocessor ignition systems are prohibitive in most small engine applications.
Moreover, in many CDI systems a somewhat high engine speed must be obtained before sufficient current pulses are generated in the charge coil and transferred to the capacitor to charge the capacitor sufficiently such that when discharged, a spark is generated across the spark gap of the spark plug. Thus, these prior ignitions systems require the engine to attain a relatively high startup speed before the ignition system is capable of producing a spark across the spark gap of the spark plug to start the engine.
Furthermore, engine overspeed is a problem in many small engine applications, such as chainsaws. It is possible for an engine to accelerate to an RPM range at which engine components and a saw blade can become damaged, such as where a load on a chainsaw is suddenly removed when the engine is operating at full throttle. Mechanical and microprocessor speed governors are typically employed to alleviate this problem, but are space-consuming and/or expensive, and often lead to unburned fuel in the engine exhaust.
Finally, it is possible during engine startup for the engine to rotate in a reverse rotational direction and for such reverse direction to be sustained after startup. Reverse startup and sustained operation may result in damage to the chainsaw and may result in a startup “kick-back” condition.
Thus, prior ignition systems are not yet fully optimized to provide a comprehensive ignition system that includes the ability to start the engine at a relatively low engine cranking speed, does not require relatively expensive microprocessor circuits, does not succumb to engine over-speed conditions, does not suffer from startup kick-back, and is of relatively simple design.